Portable rock drill apparatus adapted for underground mining and tunnelling operations are generally characterized by a movable boom for supporting at the distal end thereof an elongated drill guide or feed bar upon which a rock drill is slidably mounted. The feed bar is normally connected to the boom by way of a positioner mechanism to provide for positioning the feed bar in a wide range of positions so that a large number of blast holes may be drilled in a work surface in a particular pattern. Moreover, it is usually desired to be able to drill as many holes as possible in the workface which are parallel or nearly parallel to each other.
A number of inventions have been developed in an effort to provide a positioning mechanism which provides unrestricted movement of the feed bar with respect to the boom in order to provide the largest possible workface area in which parallel or nearly parallel holes can be drilled. U.S. Pat. No. 3,563,321, for example, discloses a positioning mechanism wherein a piston has helical splines disposed on one piston rod portion and straight splines disposed on an opposite piston rod portion. The piston is housed in a cylinder and is operably connected to a drill feed bar for rotating the same in response to axial movement of the piston.
The present invention provides improvements in rock drill positioning mechanisms including the type disclosed in U.S. Pat. No. 3,563,321 by providing a positioner mechanism which is more compact, robust, and provides for a greater drill hole workface area for parallel or nearly parallel holes than prior art positioning mechanisms.